Continuous-type centrifugal machine



3 1968 mam NIWA ET AL 3,419,148

CONTINUOUS-TYPE CENTRIFUGAL MACHINE Sheet Filed Oct. 23, 1967 ISHISABURO 0 WM T N N m m mu 0 T I m 0 w N W F 4% Sb mm, NN NW1 hmw V Q Q Nb Q iilihwi i WU QM I ii i W W I i w 0 1| I I l l II I I I \LJ M I \x Rm w Q I I. 1 I Q 3 J Dec. 31, 1968 TOSHIO NlWA ET AL 3,419,148

CONTINUOUS-TYPE CENTRIFUGAL MACHINE Sheet g of 4 Filed Oct. 23, 1967INVENTORS TOSHIO NWA AGENT SABURO OISHl Dec. 31, 1968 TOSHIO w ETAL3,419,148

CONTINUOUS-TYPE CENTRIFUGAL MACHINE Filed Oct 23, 1967 Sheet 3INVPJN'I'ORS TOSHIO NIWA ABuRo OISHI AGENT United States Patent .0

3,419,148 CONTINUOUS-TYPE CENTRIFUGAL MACHINE Toshia Niwa, Osaka, andSaburo Oishi, Takatsuki, Japan, assignors to Hitachi Shipbuilding andEngineering Co. Ltd., Osaka, Japan Filed Oct. 23, 1967, Ser. No. 677,321Claims priority, application Japan, Oct. 22, 1966,

9,891/ 66 8 Claims. (Cl. 210-413) ABSTRACT OF THE DISCLOSURE Continuousseparation of a slurry into liquid and solid phases is accomplished byproviding the basket or bowl of the inventive centrifugal machine withtwo adjoining frustoconical screen elements, the apex angle of theelement closer to the feed end of the screw being larger than that ofthe other screen element, a continuous blade being provided so as tovary the thickness of a layer of the solid phase at the demarcation linebetween the screen elements, preferably only in the area of the screenele ment closer to the discharge end.

In the sugar manufacturing industry, the centrifugal machines used inthe step of separating the sugar produced from its mother liquor are,almost without excerption, those of the batch type.

In the sugar refining industry, similarly, centrifugal machines of thebatch type are used in the aflination process as well as therefined-product sugar process. Unlike in modern chemical industrieswhere continuous centrifugal machines are rapidly replacing batch-typecentrifugals because of the disadvantages of the latter, i.e. theattrition of power and time needed for acceleration or deceleration,uneven power consumption, additional manpower and supplies required,etc., centrifugals of the batch type are still exclusively used in thesugar industry for the following reasons.

A continuous centrifugal machine of conventional design cannot produce asufficiently high centrifugal force to effect complete separation of thesolid sugar crystals from the highly viscous mother liquor, nor is itpossible to provide for a sufficient retention time during which suchcrystals may be Washed and dehydrated.

On the other hand, in the case of low-quality sugar, separation of finecrystals from the highly viscous mother liquor has been continuouslyeffected by means of a centrifugal machine of the conicalautomatic-discharge basket type. In this instance, a sufliciently highfiltration efficiency may be attained because of the centrifugal forceobtainable and the extremely thin crystal layer that is formed withinthe basket.

However, since it is impossible to provide for a sufficient retentiontime for the crystals, no sufiicient washing and dehydration effect canbe expected.

A primary object of the present invention is to provide a continuouscentrifugal machine which is so constructed that in the initial stage offiltration and separation both automatic delivery of the crystals andmaintenance of a thin cake layer are achieved, and in the subsequentstage there is controlled retention of the crystals and, at the sametime, there occurs an efficient washing of said crystals.

According to one of the main features of the invention, the centrifugalmachine comprises, in combination, a basket or bowl as well as a screwadapted to discharge the solid phase of the slurry that is centrifuged;the bowl including two frustoconical screen elements having differentapex angles, the angle of the element closer to the feed end beinglarger than that of the other screen Patented Dec. 31, 1968 element; andthere being also provided a conveying blade for the screw, only in thearea adjacent the screen element closer to the discharge end, therebyvarying the thickness of a crystal layer at the demarcation line betweenthe two screen elements.

According to other important features, slurry distribution means areprovided inside the screw conveyer, including a dispersing cone and adispersing element, for accelerating the slurry, evenly dispersing thesame over the screen elements of the basket or bowl.

Yet another inventive feature of the invention relates to the provisionof at least one supply pipe for a washing solution, in the region of thedemarcation line between the screen elements, whereby the solution isdischarged in jets against the solid-phase layer at the demarcationline.

Still another inventive feature relates to the provision of a bank orwall member inside the screw for centrifu gally accelerating the washingsolution before it is discharged from said supply pipes against thesolid-phase layer.

It is also contemplated, according to the invention, to provide meansfor recovering the spent washing solution independently from the liquidphase centrifugally separated from the slurry.

The various objects, features and attendant advantages of the presentinvention will become more apparent from the following description of apreferred, exemplary embodiment, when considered in conjunction with theaccompanying drawings wherein FIG. 1 is a side-elevational view, partlyin section, of a horizontal centrifugal machine embodying the principlesof this invention;

'FIG. 2 is a sectional detail view of the centrifugal screen elementsillustrated in FIG. 1;

FIG. 3 is a view similar to that of FIG. 2, showing a differentarrangement of screen elements that may be used in a modified embodimentof the invention;

FIG. 4 is a longitudinal sectional view showing a typical washing deviceused in the apex zone of the basket or bowl illustrated in FIG. 1;

FIG. 5 is a scross-sectional view taken along line V--V in FIG. 4;

FIGS. 6A and 6B show, in section, schematic details of conventionalcentrifugal machines, for comparison purposes; and

FIG. 60 shows, in a sectional view similar to those of FIGS. 1, 6A and6B, the centrifugal machine embodying the principles of the invention.

The invention will hereinafter be described in full de tail. Referringto FIGS. 1 to 5 and 6C of the drawings, a a basket or bowl 2 is rigidlysecured to one end of a hollow main shaft 4. A screw or conveyor 6 isrigidly secured to one end of a shaft 8 passing through said main shaft4.

The bowl 2 and screw 6 are driven by a V-pulley 10 from a conventionalmotor 12 in the same rotational direction. The respective shafts 4 and 8are driven at slightly different velocities of rotation by means of adifferential drive mechanism built into the V-pulley 10.

This mechanism is not described or illustrated herein in detail since itdoes not form the subject matter of this invention. A co-opending,co-assigned patent application entitled Centrifugal Machine withDifferential Drive, Ser. No. 677,320, filed Oct. 23, 1967, discloses amodified version of the mechanism in full detail. Some of the structuralelements of the present application are correlated hereunder, for thesake of better understanding, with the corresponding elements of theco-pending application, as follows: the bowl and the screw of thisapplication, with their respective shafts, identified by numerals 2, 6,4 and *8, correspond to elements 14, 6, 12 and 54, respectively, of theother application; the V-pulley 10 and the motor 12 have theircounterparts in the drive cylinder 22 and the motor 18 of the othercase; the slurry inlets are 36 in this case and 4 in the other, whilethe liquid outlets are numbered 60a and 8, and the solid outlets 54 and10, respectively, in the two applications.

One end of the differential drive mechanism is connected to a shaftthrough chain wheels 16 and 18, as well as a chain 20, and the chainwheel 18 on the shaft is provided with a friction plate 22 which servesas a torque limiter under overload conditions. A conventional reversiblemotor 24 is provided for the differential system of this particularcentrifugal machine. Plate 22 is an overload relief device for the chainwheel 18.

It should be noted that there is a major difference between themechanisms of the two applications. In the present case, the justdescribed elements 16 to 24, in operative connection with the V-pulley10, serve to increase or to reduce the difference between the respectivevelocities of the shafts 4 and 8, and the motor 24 serves just thispurpose. In the other application, however, a system of interconnectedarms and a braking mechanism are provided, identified by numerals 40 to50, for compensating temporary overloads between the two shafts, and forkeeping an element of the differential drive mechanism in apredetermined (preferably vertical) position. The mechanism of thepresently disclosed centrifugal machine will be described in more detailas the description proceeds.

When the rotational velocity of the main shaft 4, that is to say, of thebowl 2, is held constant and the difference with respect to the velocityof rotation of the screw shaft 8 is to be varied, it is sufficient todrive the shaft at the given velocity of rotation. This mechanism,although related to that of the other application and known in its basicprinciples, does not form part of the inventive features of thecontinuous-type centrifugal machine.

The velocity of rotation of the chain-wheel shaft is variable, and it isthus possible, even in the course of operation, to select the desireddifference between the rotational velocities of the shafts 4 and 8.

The bowl 2 consists of two frustoconical portions, one identified bynumeral 2a, having a wide apex angle, and the other, denoted 2b, havinga narrow apex angle, the demarcation line between the portions beingindicated at 26 (FIGS. 4 and 6C). Thus the bowl actually consists of twoconical portions 2a and 2b, connected in series as a single unit.

Inside the bowl 2 is built a filter wire-mesh screen 28. This screen maybe replaced by the combination of a sheet screen 28a and a backingscreen 2812, as shown in FIG. 3.

In FIG. 2, numeral 30 denotes a retaining bolt adapted to preventdislocation of the wire-mesh screen 28 in the direction of rotation.Within the bowl 2 and at the end thereof, there is a dispersing element32 (see FIG. 1), adapted to uniformly disperse the load over the screen28, the element 32 being rigidly secured to the main shaft 4.

The screw 6 is located within the bowl 2 and has a dispersing cone 34 onthe inner side thereof and concentrically therewith. The dispersing coneserves to accelerate a solution of slurry supplied through a feed-inpipe or conduit 36 which is partially inserted into the cone 34 througha narrow annular clearance and, then, it feeds the slurry to thedispersing element 32 through a passageway 38 formed in the screw 6. Thelatter has conveying blades 40 in the outer conical zone 2b only which,as aforesaid, has a relatively small apex angle.

Indicated by reference numeral 42 is a crystal washingsolution supplypipe located within the screw 6, and the Washing solution, suppliedthrough its nozzle 44 under uniform pressure, is sprayed over the filterscreen 28 through channels 46 formed in the screw 6.

The above-mentioned supply pipe 42 and the channels 46 are located alongthe demarcation line 26, between the two frustoconical cone portions 2aand 2b of the bowl 2. A liquid receiver 48 is located on the peripheralsurface of the bowl 2 and it carries a partitioning plate 50 in a planecomprising the demarcation line 26, between the truncated cones 2a, 2b,whereby the liquid receiver 48 is divided into two liquid compartments,namely 48a and 48b.

A chute 52 is made up of a metal plate lined with rubber or otherelastic material, and serves to receive, While decelerating, thecrystals extruded by the screw 6 from the centrifugal field within thebowl 2, and to guide said crystals to an outlet 54 formed in the bottomof a crystal chamber 56.

Especially when it is desired that the breakage of the crystals bereduced to a minimum, an improved shockabsorbing effect may be attainedif a flexible tube 58 filled with compressed air is disposed behind thechute 52.

In operation, the slurry to be centrifuged is fed through the pipe 36.The slurry is gradually accelerated by the dispersing cone 34, whence itis forced into the dispersing element 32 through passageway 38. Theslurry is now further accelerated by the dispering element 32 and at thesame time, is radially dispersed thereby unto the filter screen 28.Then, the slurry keeps sliding on the inner truncated cone 2a of thescreen 28, which is steeply inclined, toward the demarcation line 26,forming a thin layer.

In the process, the liquid phase of the slurry is centrifugallydischarged out of the bowl 2 through the openings in the screen 28. Theliquid phase is further guided into the liquid chamber 48a from where itis finally discharged out of the centrifuge through an outlet 60a.Outlet 60b will be described somewhat later. Thus, the solid crystalsremaining on the filter screen 28 are forced toward the demarcation line26, forming a thin layer of substantially uniform thickness.

For this reason, filtration resistance is not great, with the liquidphase of the slurry being substantially completely withdrawn from thebowl, as it passes the part of the filter screen 28 corresponding to theinner conical zone 2a of the bowl 2.

On the other hand, after passing the demarcation line 26, the crystalscannot move by themselves, but are transported under mild thrust by theconveying blade 40 of the screw 6 up to the end of the bowl 2.

It will be apparent that if the relative velocities between therotational speeds of the bowl 2 and the screw 6, and the pitch of theconveying blade 40, are previously properly set, the crystals aresuccessively delivered by the blade 40 from the demarcation line 26,thus forming a layer of uniform thickness, and at a controlled velocity,until they reach the end of the bowl 2. Thus, the time during which thecrystals remain in the bowl may be completely controlled, assuming thatthe feeding rate of the slurry is constant.

While the crystals are continuously delivered in the above-describedmanner, it a washing solution is supplied near the demarcation line 26from the supply pipe 42 inside the screw 6, the solution is forcedthrough the channels 46 unto the crystals accumulated on the screen 28,whereby the crystals are purified.

After washing the crystals, the washing solution flows through thescreen 28 into the liquid chamber 48b, to be ultimately recoveredthrough the aforementioned exit or outlet 6%, separately from the liquidphase, which has a different composition, and is discharged through theoutlet 60a, as explained before.

Since the washing is effected Where the crystal layer shifts from arelatively thin area to a thicker area, the washing solution is broughtinto more effective contact with the surface of the individual crystalsas compared with the conventional discharge of a washing solution untothe surface of. a thick layer of crystals. Stated differently, a morethorough washing effect can be had with a lesser amount of washingsolution.

In essence, the present invention provides a continuous centrifugalmachine which effects filtration of the slurry in an initial thin layerwithin the bowl 2, washing the residual crystals along the line ofdemarcation 26 between the thin layer and a thick layer that follows,and finally, dehydrating the crystals in a retention time controlled bythe conveying blade 40, whereby a considerably more efficientliquid-solid separation is attained than has been possible withconventional centrifugal machines of the continuous type.

The fundamental principles of this invention will now be compared withthe principles involved in conventional centrifugal machines, referencebeing had to FIGS. 6A and 6B for the latter. FIG. 6C shows theprinciples underlying the operation of the continuous-type centrifugalmachine according to the present invention.

FIG. 6A shows a machine, somewhat schematically, having a cylindricalbasket 2' and FIG. 6B shows one having a conical basket 2", the latterwith a single inclined surface. In both prior-art machines there isrespectively provided a conveying blade 40' throughout the entire axiallength of the baskets. Assuming that the baskets of these three machineshave the same length and maximum diameter, the machine of FIG. 6Aobviously has the maximum filtration area. However, since the basket 2'of FIG. 6A is cylindrical, the crystals cannot travel, with the resultthat the conveyance of the crystals must be commenced as soon as theslurry is fed into the machine. Thus, the conveying blade 40 agitatesthe slurry which has been only partially filtered, resulting in areduced filtration capacity.

Moreover, since the solid phase is held in contact with the blade for arelatively long time, there is an increased tendency for the crystals tobe crushed. Of particular importance is the fact that more power isrequired to drive the screw 6'. In addition, the crystal layer will beuniform over the entire length of the basket 2' so that no effectivewashing can be expected.

The machine of FIG. 6B has a somewhat smaller filtration area, and whilethe power required to drive the screw 6" is smaller by the amountcorresponding to the angle of taper of the basket or bowl, it has,otherwise, the same disadvantages as the structure of FIG. 6A. Rather,this machine is more disadvantageous in that the thickness of the layeris large in the initial stage of filtration.

If the velocity of the screw 6" is increased in order to overcome thisdisadvantage, the thickness of the layer will be decreased, but as anincreased amount of the solids comes in contact with the blade 40', thecrystals are more liable to be broken.

In the case of the inventive machine, shown in FIG. 6C, the filtrationarea falls somewhere between the areas of the prior-art machines ofFIGS. 6A and 6B. Furthermore, in the inner conical zone 2a of the bowl2, which has a relatively great angle of taper, no power is required atall to advance the crystals.

The screw 6 has no blade in the abovementioned inner conical zone 2aand, accordingly, the filtration area is not sacrificed by the presenceof such a blade. Moreover, the inner frustoconical portion 2a is sosteeply flared that the slurry travels fast, yielding an extremely thincrystal layer. Actually, therefore, the filtration speed of theinventive machine is the highest, with the additional advantage ofstabilized rotation.

The apex angle 0 as shown in FIG. 6C, of the inner frustoconical cone 2aof the bowl 2 is an important factor in the determination of thethickness of said crystal layer, and the theoretically optimum apexangle is about twice the angle of static friction between the crystalgrain and the surface of the screen 28. Thus, the apex angle preferablyranges between 100 and 50.

At the demarcation line 26 of the bowl 2, the crystal layer changes inthickness so that the washing of the crystals may be carried out moreeffectively. Furthermore, since the distance over which the crystals areforced out by the conveying blade 40 is smaller than the correspondingdistances for the other two, conventional machines shown in FIGS. 6A and6B, power consumption is drastically reduced.

In addition, as the above-mentioned distance, i.e. the outer truncatedcone 2b of the bowl 2 is mildly tapered (apex angle 0 :40 or less), thecrystals are slightly loosened as they travel, without being compacted,into a tight layer, with the result that the dehydration of the crystalsis also greatly enhanced.

Thus, when the three centrifugal machines as shown in FIGS. 6A, 6B and6C, having bowls of the same maximum diameter and the same length, aresupplied with the same amount of the same slurry, the machine of FIG.6C, according to this invention, needs only one-half to onethird as muchwashing solution as is required by the other two, conventional machines,and a substantially reduced amount of power is needed for the transferof the crystals.

Thus, the centrifugal machine of this invention is superior to theconventional continuous machines, economically speaking. Still moresatisfactory results will be attained if the washing device isconstructed as shown in FIGS. 4 and 5. The nozzle 44 of thewashing-solution supply pipe 42 discharges in the direction of rotationof the bowl 2 and screw 6 so as to avoid a collision of the washingsolution with the screw, and a ring-shaped bank or wall portion 62 (FIG.4) is disposed so that after a sufiicient pressure is built up by thecentrifugal force, the solution is caused to jet against the crystallayer on the screen 28.

When the liquid channel 46 is located axially within one pitch from theorigin of the conveying blade 40, the thickness of the crystal layerwill change from zero to maximum. Thus, the jets of the washing solutionare fully brought into contact with the surface of almost all thecrystals so that an eflicient washing result can be obtained.

More than one washing-solution supply pipe 42 may be installed inpreferred positions Within the bowl, and one of the pipes may be usedfor the purpose of promoting the drying of the cake, by the aid ofsuperheated steam.

The foregoing disclosure relates only to a preferred, exemplaryembodiment of the invention, which is intended to include all changesand modifications, as well as additions to the example described, whichare within the scope and spirit of the invention as set forth in theobjects and features outlined in the preamble, and the appended claims.

Thus, it will be appreciated, the inventive features disclosed in thisapplication may be embodied in a continuous-type centrifugal machineused in various industries for separation purposes as well although ithas been mentioned herein, as a matter of illustration, for purposes ofthe sugar manufacturing and refining industries.

What we claim is:

1. A continuous-type centrifugal machine comprising, in combination,bowl means capable of rotating at a velocity sufficiently high toseparate a slurry into liquid and solid phases; screw means capable ofrotating at a velocity different from that of said bowl means, andhaving respective feed and discharge ends; said bowl means and saidscrew means having respective drive shafts; and a mechanism forseparately driving said bowl means and said screw means at predetermineddifferent velocities, said drive shafts being substantially coaxiallysupported in conjunction with said mechanism; said bowl means includingtwo adjoining substantially frustoconical screen elements, the apexangle of one screen element which is located near said feed end of thescrew means being larger than that of the other screen element which islocated near said discharge end of the screw means; and furthercomprising a sole conveying blade associated with said screw means andbeing restrictedto the area adjacent said other screen elements, therebyvarying the thickness of a layer of the solid phase by said screw meansat the demarcation line between said screen elements.

2. The centrifugal machine as defined in claim 1, wherein said feed endof the screw means is associated with feedin means for the slurry whilesaid discharge end of the screw means is associated with discharge meansfor at least one of the liquid and solid phases.

3. The centrifugal machine as defined in claim 1, further comprisingslurry distribution means inside said screw means and including adispersing cone for centrifugally accelerating the slurry, and adispersing element for further accelerating and dispersing the slurryevenly over said screen elements.

4. The centrifugal machine as defined in claim 3, Wherein the apex angleof said one screen element ranges between 100 and 50 while that of saidother screen element is not more than 40, and wherein said dispersingcone is in the area adjacent said one screen element.

5. The centrifugal machine as defined in claim 1, further comprising atleast one supply pipe for a washing solution, disposed adjacent thedemarcation line, whereby the solution introduced by way of said supplypipe is discharged in jets against the layer of the solid phase at thedemarcation line.

6. The centrifugal machine as defined in claim 5, further comprisingmeans for recovering the Washing solution independently from thedischarged liquid phase.

7. The centrifugal machine as defined in claim 5, further comprisingannular bank means associated with said screw means for centrifugallyaccelerating the washing solution before being discharged from saidsupply pipe.

References Cited UNITED STATES PATENTS 1,589,097 6/1926 Behr 210374 X3,256,993 6/ 1966 Siepe et al. 210380 X FOREIGN PATENTS 859,429 12/1952Germany.

OTHER REFERENCES German printed application 1,151,763, July 1963,Gaessler.

REUBEN FRIEDMAN, Primary Examiner.

J. L. DE CESARE, Assistant Examiner.

US. Cl. X.R.

